Laser oscillators and laser amplifiers are most known semiconductor type laser devices, in short, semiconductor laser devices. This application is related to edge emitter semiconductor lasers, wherein the amplification by stimulated emission (laser effect) is produced along a device that is millimeters long and the emission exits through mirrors, also known as windows, at the ends of the device. It is proper to define the laser resonator between two mirrors and the laser amplifiers between two windows. Nevertheless, the exit mirror of a laser device is often named the exit window or front window. It is known that the active region projection on the exit window facet is the element of the semiconductor type laser device that is most sensitive to degradation. This is the place where Catastrophic Optical Degradation (COD) and important gradual degradation processes occur. COD processes exist also at the rear mirror but are less intense because the optical power is lower. These degradation processes represent important factors that determine the limits for the operation of these lasers at high power and at high power density of the radiation that traverse the exit window. The power density is defined sometimes as power flux per unit window area and other times as linear power density, i.e. as the power flux per unit width of the exit aperture. The catastrophic degradation is practically instantaneous when the power density of the radiation, emitted through the active region at the window facet, overpasses certain threshold values. The values for the power density of the emitted radiation that passes through the active region and produce catastrophic degradation are, in a great extent, material characteristics. In some cases, the gradual degradation starts from the windows, having in the end, after a period, the same effects as the catastrophic degradation, i.e. the irremediable destruction of the windows and of the laser. To avoid degradation, it is recommended to operate the laser at power levels and power density levels lower than the catastrophic degradation levels.
The catastrophic degradation is produced with the contribution of electronic states at the exit windows facets, surface states that modify the distribution of the electrical potential and the light absorption phenomena in the superficial layer at the semiconductor material-external medium interface. Oxygen is the most deleterious element that produces a great amount of surface states. To remedy the effects induced by these surface states several solutions for obtaining less sensitive “windows” for semiconductor laser were imagined:                1) The solutions that address the layered semiconductor laser structure itself by lowering the ratio of the emitted power through the mentioned most sensitive window element relative to the total emitted power, ratio also known as active region confinement factor Γ;        2) The solutions that address the passivation of the exit window facets;        3) The solutions that address the introduction of an entire end segment with a modified layered structure situated longitudinally between the main amplifying segment and the exit window, end segment that protects the exit window.        
In this section, the existing solutions related to the point 1) and 3) are discussed as being closely related to the subject matter of the present invention. These solutions avoid the cumbersome methods related to mirror facets passivation.
Among the solutions which address the lowering of the confinement factor Γ, the closest is a solution that utilizes an asymmetric double waveguide transversally comprising an active waveguide and a passive trapping waveguide situated at one side of the active waveguide, wherein part of the radiation emitted in the active region is captured by the passive trapping waveguide. The waveguides are limited by cladding layers, known also as confinement layers, which limit the transversal extent and may contain the exponential decay of radiation distributions. The double waveguide is designed to amplify preferentially the fundamental mode with the same phase over the both waveguides.
The active waveguide contains the active region, consisting of one or more quantum wells separated by quantum barriers or may have the width larger than what is commonly understood by quantum well. The active region is usually contained in an active waveguide or can form alone, intrinsically, the active waveguide.
Such constructions with double waveguides were disclosed in the U.S. Pat. No. 6,522,677. In this publication, structures with confinement factor less than 0.015%, even for large active regions, wider than 10 nm, have been described. Reduced confinement factor reduces the modal gain and allows the use of long waveguides, longer than 2 mm. The reduced confinement factor, reducing the power density traversing the output window through the active region, also increases the total emitted power for which the catastrophic degradation occurs. For such constructions, better and better outcomes were obtained, described for example in non-patent literature documents by Petrescu-Prahova et al. [1] and [2]. At very high power density, the catastrophic optical degradation remains an issue.
The transversal structures with double waveguide mentioned in above are well suited for being used in longitudinal structures comprising at least two longitudinal segments: a main one, where amplification of the radiation occurs, and an end one, adjacent to the front window, where the active region was completely removed and replaced with a semiconductor material with suitable optical properties. Such construction was described in U.S. Pat. No. 6,272,161. The essential fact for the construction of this patent is that the overlapping integral I00 between the functions describing the radiation distributions of the transversal fundamental mode in the two mentioned segments has values very close to unity, what reduces the coupling loss L when crossing the interface, approximately equal to L=1−|I00|2. The advantage of this construction is that no radiation exits through the active region at the output window because the active region was completely removed. In this publication, the etch-regrowth method for construction of end segment is proposed. All upper layers, including the active region, were removed by etching from a segment adjacent to the window and after that were replaced with other layers with suitable optical properties by regrowth. The construction described in U.S. Pat. No. 6,272,161 has the disadvantage that the etch processes followed by regrowth are difficult to be processed and, also, that the degradation processes can be moved from front window toward the interface between the main segment and the end segment, with the participation of etch defects remaining at the extremity of the unetched active region.
A construction transversally based on a symmetric structure and longitudinally consisting of at least two segments was described in non-patent literature document by Baoxue et al. [3]. The construction is presented schematically in FIG. 1. The transversal structure has a core waveguide 01, a lower cladding 02 and an upper cladding 03. The structure has the same composition and thickness for both claddings. Longitudinally, the construction has a main segment 020 and an end segment 021. The end segment is formed by removal of part of the upper cladding in a single step, in an etch process that does not touch the active region. The refractive index profile for such a structure is shown schematically in FIG. 2A. FIGS. 2B and 2C shows schematically two other transversal structures used for the construction of two longitudinal segments.
It is desirable that removal of part of the upper claddings in the construction of FIG. 1 would have had the effect of reducing the confinement factor Γ for the fundamental mode, which reduces the power density that passes through the active region at the exit and also increases the total emitted power for which the catastrophic degradation occurs. A complementary effect is that the coupling integral between the fundamental modes of two segments becomes less than 1. As the etch depth of the upper cladding 03 increases, the overlapping coupling integral I00 decreases and the coupling loss L increases. It is essential that the coupling integral between the fundamental modes of both segments to remain close to unity. It is generally assumed that the deeper is the removed part of the upper layers the greater is the confinement factor reduction for the fundamental mode, but this is not always true. The construction proposed in non-patent literature document [3], with a symmetric transversal structure, has the disadvantage that the confinement factor Γ firstly increases as the etch depth increases and the loss is already too significant when the confinement factor Γ starts to appreciably decrease. The relation between the variation of the Γ factor and the increase of coupling loss calculated for the symmetric case is shown in FIG. 3 for the symmetric 808 nm structure described in Table 1, row (a).
Another transversal structure for a construction of FIG. 1 is described in U.S. 2011/0317730 A1. It is based on an asymmetric structure, with a thin upper cladding 03 having a low refractive index and a thick lower cladding 02 having high refractive index. The profile of the refractive index is shown schematically in FIG. 2B. In FIG. 3 is shown a better, acceptable relation between the decrease of the Γ factor and the increase of coupling losses, i.e. the asymmetric curve is closer to the Γ axis than the symmetric curve. An exemplifying 808 nm asymmetric structure is described in Table 1, row (b).
Another transversal structure for a construction of FIG. 1, also proposed in U.S. 2011/0317730 A1, includes a double waveguide. FIG. 2C shows schematically the profile of the refractive index of a transversal double waveguide structures. A further improvement in the relation between the decrease of the Γ factor and the increase of coupling loss is shown in FIG. 3, for an 808 nm exemplifying double waveguide structure described in Table 1, row (c).
The cases B and C of FIG. 3 show that the reasonable reduction of confinement factor Γ should not be lower than 60%-70% of the value Γ0 in the main segment, due to the very high values for coupling losses afterword. Constructions described in U.S. 2011/0317730 A1 have the limitation that the decrease of the confinement factor for the fundamental mode in the end segment adjacent to the front window, compared to the confinement factor of the main segment, is relatively low, only ×1.3-×1.5. This decrease is limited by the enhancing of the complementary effect, namely the decrease of the fundamental mode coupling, when the etching is too deep.
In an embodiment of the present invention, a substantial higher decrease of the confinement factor Γ for the fundamental mode both at the front exit window and at the rear window, usually a reflecting mirror, is obtained. Both windows are conventionally called exit windows or in short windows. The substantial decrease of the confinement factor Γ at windows by the use of end segments described in this invention is obtained with better coupling between fundamental modes of the two sections with the avoidance of high transfer losses to higher-order modes that could reduce significantly the semiconductor laser efficiency. By this construction, a significant increase of total power needed for destroying windows by catastrophic degradation is obtained. By the excellent window protection when semiconductor laser operates at high power, the possibility of catastrophic degradation is reduced and the gradual degradation processes are considerably slowed.
It is the object of this invention to use planar asymmetric, double waveguide, low confinement semiconductor laser layered structures for the main, amplifying segment and to use modified transversal layered structures for the end segments for protecting mirror facets.